The La Liga side have thrown a spanner into the works at the eleventh hour because they cannot be seen to be willingly selling their best home-grown player in the eyes of their Basque fans for political reasons.

However United can get round this impasse by giving Herrera's camp the £29million fee to give to Bilbao to activate his release clause.

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Technically, Herrera, 24, would then be a free agent and able to join United.

Javi Martinez had to jump through similar hoops two years ago when he left Bilbao to join Bayern Munich in a record £31.6million deal.

Bayern gave his people the money to trigger his release clause at Bilbao, allowing him to move to Bavaria.

United remain confident Herrera's transfer will be concluded soon and that he will become Louis van Gaal's first signing.